a lot depends on what types of corals you have. Soft corals (mushrooms, some leathers, some polyps) require much less lighting than SPS (acropora, montipora, etc) corals do. Then there are LPS (torch, brain, etc) that have anywhere from medium low to high lighting requirements.
So a lot depends on the type of coral and what type of lighting you have.
Sorry I can't give you a better, more straight forward X amount of hours answer.